![1boy andou_jurai animation_paper closed_eyes commentary crying inou-battle_wa_nichijou-kei_no_naka_de key_frame messy_hair mouth official_art open_mouth partially_colored production_art production_note screaming short_hair sketch teeth trigger_(company) yellow_background rating:s score:0 post #1817805](/cdn_image/preview/d6/c2/d6c2e74a1ca6efa66f5fb3d49176f30b.jpg)
From Wikipedia:
"A key frame in animation and filmmaking is a drawing that defines the starting and ending points of any smooth transition. The drawings are called "frames" because their position in time is measured in frames on a strip of film. A sequence of keyframes defines which movement the viewer will see, whereas the position of the keyframes on the film, video or animation defines the timing of the movement. Because only two or three keyframes over the span of a second do not create the illusion of movement, the remaining frames are filled with inbetweens."
In other words, it is raw still shot that was used directly for the producing the animation. This covers parts, or even all of, the draft sketch to test out plot flow, pre-color scenes, cel that was used for the animation, or complete film still. It is primarily used to complete the animation, since key frames themselves do not show any movement; it is important however, if not most important in developing animations.
It is not necessarily same thing as screencap, which captures the final material in presented medium. Key frames are generally strictly internal-use only.
See also official art, production art.